1. Field of the Invention
The present invention relates to a test device for locating first and second fault types in a two-lead line, wherein the first fault type includes cut-offs, breaks or open circuits and the second fault type includes short circuits, insulation faults or low resistance shunts between one of the line leads and the ground or between the two line leads.
2. Description of the Prior Art
U.S. Pat. No. 3,235,677 describes a fault locating test device which comprises a first dipole connected in shunt with two leads of a communication line. The first dipole comprises a resistor and a rectifier or diode which are series-connected. One of the line ends is connected to a load while the other end is selectively connected to a resistance measuring apparatus.
Line breaks are located with respect to the first dipole by detecting a leakage current which intentionally flows through the diode of the first dipole. The presence of such a shunt diode requires measurement voltages applied to one end of the line to be of a predetermined polarity that is opposite to the polarity of the voltages applied to the line end under normal operating conditions. This drawback requires strict surveillance to ensure that the polarity of the applied voltages is maintained, particularly when working on the line.
Over and above the foregoing, the characteristics of this device are such that it is incompatible with the majority of existing telephone subscriber line test and measuring apparatus. Modifications or additions to these apparatus found in any telephone exchange thus become an extremely expensive necessity in view of the number of apparatus in question.
Furthermore, the distribution between intentionally induced leakage currents and natural leakage currents in certain imperfectly insulated lines is rather fine and makes locating an open circuit in a line fitted with such test device illusory.